Conventionally, spot welding techniques of pressing electrodes mounted on and held to tips of a weld gun to steel plates, applying a pressure thereto, and causing energization to provide resistance heating to the steel plates to weld them together have been used in automobile production lines. The spot welding techniques ensure reliability of products with proper management of current values during welding, weld times, applied pressure force, and conditions of tips of the electrodes. Among these elements, the tips of the electrodes ensure welding quality most stably if they have a true circular shape and do not include waste materials attached thereto. However, after the welding is done a predetermined times, the tips of the electrodes do not have the true circular shape due to wear, or oxide coatings and the like are attached to the tips, resulting in deterioration of the tip condition, and if the welding is continuously performed with the tips deteriorated, the quality of the welded parts is not ensured. Therefore, it is necessary to dress the deteriorated tips of the electrodes so that the tips are a proper condition, and a proper management of the dressed condition is needed. For example, Patent Document 1 shows periodically capturing images of electrodes by a camera from a side of a weld gun with the electrodes mounted on and held to tips of the weld gun, calculating the diameters of tips of the electrodes by using the captured images, and checking conditions of the tips of the electrodes to compare and determine whether the electrodes are in a normal condition to manage the condition of dressing in the electrodes.